Did a similar thing last year but without the 100mm shelf around.
In other words made a panel such that face of tiles is roughly flush with
lip edge of bath.

The way I got round the access issue was to cut access squares in the
plywood panel, but keep the cutout sections. I then fitted spring catches
to the cutout sections such that they click back into place flush with main
panel (the catches used were those that you used to see on old fashioned
larder doors - not sure of the name).

Then tiled over panel making sure that tiles over the removable panels were
stuck to the removable bit only and not the main panel if they overlapped.
Made sure that the bottom edge of the lowest tiles were just above the
lowest edge of the removable panel.
Cut a notch in the bottom of the removable panel to form a slot when in
place.
Left a slightly thicker grout joint below these tiles.
Used removable gasket material rather than grout around the edges of the
tiles stuck to the removable panel.

So if the worst happens:

Remove gasket-grout.
Insert bent wire "key" into slot below removable panel.
Rotate wire key thro 90 degrees to grab back of panel.
Pull firmly trying not to snag tile edge.
Panel come off complete with fixed tiles.
When finished, push back into place and re-"grout".

Phil